In a letter submitted Friday afternoon to internal investigators at the Nuclear Regulatory Commission, a whistleblower engineer within the agency accused regulators of deliberately covering up information relating to the vulnerability of U.S. nuclear power facilities that sit downstream from large dams and reservoirs.

The letter also accuses the agency of failing to act to correct these vulnerabilities despite being aware of the risks for years.These charges were echoed in separate conversations with another risk engineer inside the agency who suggested that the vulnerability at one plant in particular — the three-reactor Oconee Nuclear Station near Seneca, S.C. — put it at risk of a flood and subsequent systems failure, should an upstream dam completely fail, that would be similar to the tsunami that hobbled the Fukushima Daiichi nuclear facility in Japan last year. That event caused multiple reactor meltdowns.

In the letter, a copy of which was obtained by The Huffington Post, Richard H. Perkins, a reliability and risk engineer with the agency’s division of risk analysis, alleged that NRC officials falsely invoked security concerns in redacting large portions of a report detailing the agency’s preliminary investigation into the potential for flooding at U.S. nuclear power plants due to upstream dam failure.

In addition to the Oconee facility, the report examined similar vulnerabilities at the Ft. Calhoun station in Nebraska, the Prairie Island facility in Minnesota and the Watts Bar plant in Tennessee, among others.

Perkins was the lead author of that report, which was completed in July of 2011 — roughly four months after an earthquake and subsequent tsunami flooded the Fukushima Daiichi nuclear power plant in Japan, cut off electric power to the facility and disabled all of its backup power systems, eliminating the ability to keep the reactors cool and leading to a meltdown.

The report concluded, among other things, that the failure of one or more dams sitting upstream from several nuclear power plants “may result in flood levels at a site that render essential safety systems inoperable.